The radioactive noble gas radon is a health hazard when accumulated indoor, from a radiation protection point of view. However, radon is being also studied in the environment as a useful tracer to investigate atmospheric processes, to improve Atmospheric Transport Models (ATM) or indirectly retrieve fluxes of greenhouse gases. To increase the comparability for both radiation protection measurements as well as those used for GHG modelling, traceability to the SI is needed for radon release rates from soil as well as its concentration in the atmosphere. Radon flux relates to the transfer process of radon activity from soil to the atmosphere per square meter and second (Bq m-2 s-1), whilst radon activity concentration is the amount of activity of radon in the atmosphere per cubic meter (Bq m-3). Traceability to a primary standard does not currently exist for these quantities, which limits the usability of their measurements and their harmonization across Europe.

Climatic Atmospheric Monitoring Networks (AMN) like the pan-European Integrated Carbon Observation System (ICOS, http://www.icos-cp.eu, last access: 2 March 2022​​​​​​​), are infrastructures that operate GHG monitoring stations and are also including atmospheric radon measurements based on different techniques (Schmithüsen et al., 2017; Grossi et al., 2020). The radon data from these networks can be used, among others, to improve ATM, to study atmospheric processes and to indirectly estimate GHG fluxes by the Radon Tracer Method (RTM) (e.g., Van Der Laan et al., 2010; Levin et al., 2011, 2021; Vogel et al., 2012; Wada et al., 2013; Grossi et al., 2014, 2018), which uses the correlation between GHG and radon activity concentrations assuming a known radon flux over the footprint area. These measurements need significant improvement in terms of the accuracy of the environmental radon activity concentrations which ranges usually between 1 and 100 Bq m-3 to be able to provide robust data for the use in the RTM and thus minimize the uncertainties on the retrieved GHG fluxes. At present, commercial radon monitors currently available on the market present a high uncertainty when measuring radon activity concentration below 100 Bq m-3 as reported by Radulescu et al. (2022). Radon flux data, useful to validate radon flux maps and models, also need a robust traceability chain and a related infrastructure as explained in detail by Röttger et al. (2021).

Similarly, for radiation monitoring, all European countries have installed networks of automatic radiation dose and airborne contamination monitoring stations and report the information gathered to the EURDEP, thus supporting EU member states and the EURATOM treaty (Sangiorgi et al., 2020). Currently, monitoring information on dose rates is collected from 5500 automatic surveillance systems in 40 countries, however, urgently needed data on outdoor (atmospheric) radon activity concentrations is not yet collected due to a lack of harmonization between research monitors able to measure accurately at the low levels encountered in the environment. Furthermore, accurately detecting radioactivity from nuclear and radiological emergencies using ambient dose rate measurements relies on rejecting false positive results based on particulate radon progeny washed from the atmosphere by rain. Therefore, improving early warning detection systems for radioactivity requires greater accuracy in determining environmental radon activity concentrations.

The project traceRadon, started in June 2020 joining together the atmosphere, climate and the environmental radiation scientific communities under the umbrella of EURAMET, the European Association of National Metrology Institutes. At the very moment 18 partners and 12 collaborators are working towards a European harmonisation within the goals of this project, which consists of four technical Working Packages (WP):

WP1 aims to develop traceable methods for the measurement of outdoor low-level radon activity concentrations in the range of 1 to 100 Bq m-3​​​​​​​ with uncertainties of 10 % (k=1) to be used in climate and radiation protection networks. These methods will include two new traceable 222Rn emanation sources below 100 Bq m-3, a traceable transfer instrument calibrated with these new sources and a calibration procedure suitable to enable a traceable calibration of environmental atmospheric radon measurement systems in the field;

In the context of Earth system research, more detailed investigations of exchange processes of fluxes of GHG have increasingly come into focus in recent decades. Continuous measurements of GHG fluxes (CO2, CH4, N2O) between different ecosystems and the atmosphere are extremely heterogeneous because they are mainly dominated by local-scale changes in their sources and sinks, chemical reactions, as well as meteorological conditions. Local GHG flux measurements on the meter to kilometre scale are therefore often not representative of regional exchange rates on the scale of several kilometres. Here, the naturally occurring noble gas radon, which is emitted from continental soils but not from water bodies (such as oceans, seas, lakes, etc.) can help to overcome these difficulties by using it as a tracer for boundary layer mixing processes.

Similar to other gases which have distributed sources close to the ground radon accumulates, is mixed, and disperses within the atmospheric boundary layer. If the radon flux from the soil is known, the correlated increase of radon and GHG concentration can be used to estimate the GHG flux. Radon emissions from the land surface are influenced by the geophysical properties of the soils and its only sink is its radioactive decay (T1/2≈3.8 d). Radon is produced continuously by radioactive decay of radium (226Ra) in the soil, but its exhalation rate to the atmosphere is influenced by geophysical propriety of the soils (porosity, permeability, etc.) and by climatic conditions (rain, snow, temperature, etc.) (López-Coto et al., 2013; Karstens et al., 2015). Compared to the large spatiotemporal heterogeneity of GHG fluxes, radon emissions from ice-free land are often assumed to be more homogeneous (or at least uncorrelated with changes in the GHG fluxes on local spatial scales and diurnal time scales) and therefore easier to model. However, the reliability of the GHG flux estimates depend critically on the accuracy and representativeness of the value used for the radon flux in the footprint of the measurement station (Grossi et al., 2018; Levin et al., 2021) and hence requires a careful validation of the radon flux maps. Similarly, regional atmospheric transport models, such as those used in weather forecasting, can be validated and/or improved using radon as a transport tracer (e. g. Olivié et al., 2004; Koffi et al., 2016).

Within traceRadon, new chains of traceability are developed with respect to the activity concentration in the atmosphere as well as for the radon flux from the soil. Traceability in the metrological sense means that the measurement procedure is linked to the SI system of units in an absolute sense, in the expected measurement range (Röttger and Honig, 2011). An important part of the calibration and the measurement is the assignment of an uncertainty, which includes all individual uncertainty contributions (statistics of the measurement, uncertainty of the calibration, intrinsic background of the instrument, etc.). Because radon is so well suited as a tracer for climate monitoring, very sensitive radon monitors have been developed for this purpose during the last years by research entities and are running at different European atmospheric stations (Whittlestone and Zahorowski, 1998; Levin et al., 2002; Grossi et al., 2012; Griffiths et al., 2016). Sensitive means that, even at small activity concentrations down to few Bq m-3, small changes can be observed on an hourly basis. The ability to observe relative changes with a small uncertainty, however, says nothing about the uncertainty of the absolute value. The lack of traceability so far prevents radon measurements from being compared independently of the devices used at the different AMN stations as well as at radiological stations. Since the measuring devices are usually permanently installed at one location, the aim of the project is to develop a transportable calibration option for them.

The traceability for radon flux will not be covered by this publication in detail. But currently it can be stated that based on the observed needs, a radon “exhalation bed” to be used as a calibration facility has been designed and constructed. Several experiments were carried out with the aim for testing the reliability of an exhalation bed and to calibrate radon flux systems under different environmental conditions. Up to now different radon flux systems were simultaneously tested over the bed surface under dynamic or static conditions. The evaluation is not finished but shows promising results.

Atmospheric radon measurements are used in numerous atmospheric studies due to radon's unique physical and chemical characteristics (e.g., inert gas, simple source, and only radioactive decay as a sink). Among these applications, radon measurements in combination with GHG measurements can be used to indirectly estimate emission fluxes using the RTM (Dörr et al., 1983). The RTM uses atmospheric measurements of radon and the GHG of interest and measured, or modelled, values of radon fluxes. The RTM is based on the assumption that the nocturnal lower atmospheric boundary layer can be described as a well-mixed box of air where the changes in the radon and GHG concentrations are only due to their respective fluxes' contribution into the atmospheric volume. The change in atmospheric concentration conditions can be described as:​​​​​​​ 1ΔCiΔt=∫t0t1h(t)-1jidt=h-1‾ji‾ Where h(t) is the time dependent mixing height, ji(t) represents the surface flux of a species i, and ΔCi is the departure of its concentration relative to initial background levels for a given observation time Δti.e.t1-t0 The overbar indicates that both mixing height and net surface flux of the catchment area are averaged for the observation period. This version of the box model assumes a constant h-1 over the time window and neglects the radioactive decay of radon over the integration time period. Since the Eq. (1) can be obtained for both gases (e.g., 222Rn and GHG), the mixing layer height (which may be unknown) and time step cancel after taking the ratio yielding: 2jx‾=ΔCxΔC222RnjC222Rn‾ Where ΔCx and ΔC222Rn represent the concentration enhancements of species x (in this case one of the GHG) and radon above their respective background levels for a given observation period. The jx‾ and jC222Rn‾ represent the mean surface fluxes of 222Rn and GHG of the sources within the catchment footprint.

Radon flux, used for RTM applications in the literature, was usually taken as a constant value obtained by experimental measurements carried out close to the atmospheric station where radon and GHG atmospheric measurements were obtained or from the output of radon flux models in the grid containing the station location. Grossi et al. (2018) proposed for the first time to use within the RTM a potentially different “effective” radon flux influencing the atmospheric station each night of the studied time series. This value, changing for each night episode, was calculated by coupling the radon flux model, with the resident time map of the air masses over the region of interest. This last map was calculated using the backtrajectories with an atmospheric transport model (in their case the ECMWF-FLEXPART model (version 9.02) (Stohl et al., 1998)). This new methodology also accounts for the spatial variability of the radon flux within the footprint of the station which could be a significant problem for the RTM application as reported by Levin et al. (2021).

RTM is therefore based on both radon and GHG atmospheric concentration time series. However, so far RTM applications studies were carried out using: (1) local radon activity concentrations measured with different techniques and without being supported by a robust metrological traceability chain; (2) using radon flux value from not validated radon flux models or from experimental observations not supported by a proper petrology; (3) individual RTM without a specific application protocol. Thus, so far, GHG fluxes obtained by RTM at different locations and by different scientists cannot be compared because the RTM has not yet a unique application protocol. Firstly, there is a need to develop a traceability chain for radon instrument calibration from lab to field and secondly a standard protocol for RTM application is required where it should be specified, among others, the length of the nocturnal window to be analysed, the threshold of the linear correlation between radon and GHG during their increase, how to calculate the radon flux over the studied episodes. Both goals follow into the scope of the traceRadon project.

As described by Grossi et al. (2018) and Levin et al. (2021), it is very important to understand how the different selections in the application of this method will influence its result. Thus, there is a huge need to develop and test an agreed standard procedure for application of the RTM on different AMN sites. Such procedure would be a starting point for how the climate community could utilise radon measurements on local and regional scales from operational networks such as ICOS. The project traceRadon will provide main steps in overcoming these weaknesses. The first steps in this direction are described in section 6 and 7 by developing new sources and specifying requirements for transfer standards. Moreover, this procedure is important for GHG emission estimates that support national reporting under the Paris agreement on climate change.

The Council Directive 2013/59/EURATOM, published in 2014, contains new requirements for radon protection. Article 103 (3) requires Member States to identify areas where the radon activity concentration (annual mean) is expected to exceed the relevant national reference value in a significant number of buildings. In these areas, Member States are requested to establish programmes to carry out radon measurements in workplaces that are located at ground or basement level (Article 54 (2)). The establishment of Radon Priority Areas (RPA) is therefore an important step in the implementation of the Directive and in the radon action plans of the Member States.

Maps directly or indirectly related to soil 226Ra content existed in many member countries long before the implementation of the European Directive (Dubois, 2005​​​​​​​). They are an important part of national radon strategies as a tool for prioritization of radon protection measures (e.g., structural precautions, radon measurements, awareness campaigns for decision makers and the public). Radon maps and RPAs can be produced and displayed using very different methods, depending on the intended use of the map and the available data (Bossew, 2018). Many maps and the setting of RPAs are based on indoor radon measurements in residential buildings, sometimes linked to information on building characteristics (e.g., Austria, see Friedmann, 2005). Other countries base the determination on data on radon in soil gas (e.g., Czech Republic and Germany, see Neznal et al., 2004 and Kemski et al., 2001) respectively. Another method for characterizing an area in terms of its radon potential is to link different radon-relevant parameters (e.g., radon in soil air, soil permeability, concentration of radionuclides in soil, soil characteristics). This is often referred to as the geogenic radon potential, and methods for this have been further developed thanks to the MetroRADON project (e.g., the Radon Hazard Index, RHI) (Bossew et al., 2020).

The Joint Research Centre of the European Commission (EC-JRC) has been collecting data on radon and natural sources of radiation from European countries and from available European databases. As results, the European Atlas of Natural Radiation (Tollefsen et al., 2019) contains a collection of maps showing the levels of natural sources of radiation in different parts of Europe. Moreover, it provides reference values for natural sources of radiation across Europe and makes harmonised datasets available to the scientific community and national authorities. This also serves the purpose of raising awareness about natural radioactivity in the public. The Atlas publication is available in digital format and as a printed version, in addition an online version is regularly updated when new data is available (https://remon.jrc.ec.europa.eu, last access: 2 March 2022). Radon outdoor air and radon flux maps have not been developed in the Atlas project so far, because the data were not yet available, and especially not robust and comparable. In the traceRadon project, for the first time an evaluation whether and how these data can be used in future methods and models for radon mapping and determination of RPAs is performed. Consideration of dynamic soil parameters (e.g., soil moisture) on radon transport processes, and thus on radon activity concentrations in soil air, may contribute significantly to characterization of radon potential. Within this project experimental calibrated data of atmospheric radon activity concentrations and radon flux as well as dose rate and spectrometry data (Röttger and Kessler, 2019​​​​​​​) will be used to validate available radon flux models and inventories (e.g., Szegvary et al., 2009; Karstens et al., 2015). Finally, it is planned to make the radon outdoor air monitoring stations available online similar to the local gamma dose rate via EURDEP.

To start the development of new sources for the calibration needs (Linzmaier and Röttger, 2014​​​​​​​), a literature study of currently available radon sources for the calibration of instruments capable of measuring radon activity concentrations below 100 Bq m-3 was performed. Environmental parameter ranges were evaluated and a suitability list for in-field calibration was defined. That led to new needs for these characteristics, specifically the ability to produce reference atmospheres at the ambient levels as well as high stability concerning environmental parameters for potential in-situ applications. Currently, no sources which meet both criteria are available and so alternative sources had to be developed for the traceRadon Project: low activity emanation sources. Two different principles were applied:

a radon emanation source is created from an emulsion of salts of fatty acids in silicone rubber, formed from a weighed standard solution of 226Ra. Traceability of the 226Ra activity is established by weighing and γ-spectrometry: Using a stainless-steel cylindrical case with valves and aerosol filters, applying ultra-dried air and a mass flow controller with additional humidifier, to control the dilution of the activity concentration, a time-stable radon activity concentration is achieved. This principal was already successfully applied in MetroRADON (Fialova et al., 2020);

In the scope of traceRadon, the two different sources will be compared regarding their applicability for in-situ calibrations, their stability with respect to environmental parameters as well as the match between their indicated 222Rn emanation values. Note, that the line of traceability for the two sources is very different: (1) Achieves traceability to the SI by the traceability of the weighing process and is thus traceable to the national 226Ra standards of the CMI (Fialova et al., 2020) and by characterization of the emanation by γ-ray spectrometry in comparison to simulated results. On the other hand, (2) achieves primary definition of the 226Ra activity, traceable to the meter and the second by employing defined solid-angle α-particle spectrometry and characterizes the emanation by γ-ray spectrometry in comparison with a sealed reference source (Mertes et al., 2020). While the stability with respect to environmental changes is assumed for source type (1), it can be at least retrospectively monitored for source type (2) using the outlined algorithm. Therefore, consistent calibration results with each source will provide significant confidence concerning the correctness of the applied methods. This may only be achieved by pursuing different source construction techniques with independent traceability chains.

With the new sources traceable calibrations are feasible, but as a prerequisite an instrument capable to operate in that range of activity concentration is needed. Nowadays a variety of commercial radon monitors are available on the market based on different measurement techniques. Additionally, radon monitors have also been developed at some research institutes. A literature review of the currently available continuous radon monitors capable of measuring activity concentrations below 100 Bq m-3 was carrying out during the first months of the traceRadon project. A summary of it is reported in Table 1 with the main characteristics of the monitors as declared by the manufacturer. Often the trade-off between commercial and research-grade radon monitors is a reduction of portability for an increase in accuracy at low-level measurements. Note that the nominal sensitivity in Table 1 is the one declared by the manufacturers and it is considering only the uncertainty of the total counts in the region of interest of each instrument for radon measurement and/or calculation. This information only gives a first order of magnitude to compare the different monitors but other factors influencing the total uncertainty of the measurement such as calibration factor uncertainty, background uncertainty, interference factor uncertainty, uncertainties of correction factors applied to the different monitors due to their measurement method (temperature, water content, pressure, time response, deconvolution, etc.) are not taken into account (Radulescu et al., 2022). In addition, a desired matrix of properties usable for radon activity concentration monitors for use as a transfer standard in the field are shown in Table 2. They also provide a mark to be accomplish for atmospheric radon measurements at AMN from the point of view of the traceRadon project. The matrix is available to guide developers and producers in instrumental design for climate observation networks, since currently no IEC standard for this application exists.

Atmospheric measurements of radon activity concentrations can become a pillar in the assessment and improvement of ATM. For this aim, the metrology of the radon activity concentration has been improved by the development of a new traceability chain in extension of Pierre et al. (2021), a new data analysis for the online source application and by the implementation of a matrix with recommended properties for the in-field application of a transfer standard radon monitor for atmospheric measurements.

This is also the first step to overcome the current weaknesses in the RTM. The next step within traceRadon will be to establish a traceability chain for radon flux measurements as well.

Useable dynamic radon activity concentration and radon flux maps will be provided through the Radioactive Environmental Monitoring web portal (REMon, https://remon.jrc.ec.europa.eu, last access: 2 March 2022) operated by the Joint Research Centre of the European Commission and through the ICOS Carbon Portal. This new data will be linked to already established data from EURDEP (European Radiological Data Exchange Platform) and EANR (European Atlas of Natural Radiation) and will be made available to scientists, policy makers and end users. Concerning radiation protection, the focus is on the improvement of methods for the identification of Radon Priority Areas and the identification of radon wash-out peaks from total gamma dose rate data.